NASA image shows why San Francisco is foggy

Image of San Francisco taken on August 16th. Image by NASA. Click to enlarge.

The short answer to why San Francisco, California is foggy? The Pacific Ocean’s marine layer. A new image by NASA’s Earth Observing-1 satellite shows the marine layer—cool, heavy air produced by a colder ocean surface meeting warmer air—encroaching on the metropolis. Western winds push the marine layer over the city, which brings dense cloud cover over the city, and often engulfs buildings, bridges, and people in fog. Fog is defined as suspended droplets that limit visibility to 1,000 meters (3,000 feet), anything less dense is technically considered mist.

This weather pattern is most common in the summer due to shifts in the California Current which brings an upwelling of frigid, deep water to the surface of the ocean. Once the cold water meets the warmer air, the marine layer is formed, blanketing San Francisco in its iconic fog.

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